1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor substrate provided with a single crystal semiconductor layer over a base substrate having an insulating surface. In addition, the present invention relates to a semiconductor device manufactured using the semiconductor substrate.
2. Description of the Related Art
In recent years, an integrated circuit using an SOI (silicon on insulator) substrate in which a thin single crystal semiconductor layer is formed on an insulating surface, instead of a bulk silicon wafer has been developed. By utilizing characteristics of a thin single crystal silicon layer formed over an insulating film, transistors formed in the integrated circuit can be electrically separated from each other completely. Further, each transistor can be formed as a fully-depleted transistor, and thus a semiconductor integrated circuit with high added value such as high integration, high speed driving, and low voltage consumption can be realized.
As a method for manufacturing an SOI substrate, a hydrogen ion implantation separation method in which hydrogen ion implantation and separation are combined is known. A method for manufacturing an SOI substrate using a hydrogen ion implantation separation method is briefly described below. First, by implantation of hydrogen ions into a silicon wafer serving as a substrate for separation by an ion implantation method, an ion-implanted layer is formed at a predetermined depth from the surface. Then, the silicon wafer into which the hydrogen ions are implanted is bonded to another silicon wafer with a silicon oxide film interposed therebetween (bonding). Then, through heat treatment, the ion-implanted layer serves as a cleavage plane and separation into a thin film occurs in the silicon wafer to be separated into which hydrogen ions are implanted. Accordingly, a single crystal silicon film can be formed over the silicon wafer to be separated. The hydrogen ion implantation separation method is also called a Smart Cut (registered trademark) method.
A method in which a single crystal silicon film is formed over a base substrate made of glass by a hydrogen ion implantation separation method has been proposed. For example, in Reference 1, a separation plane is mechanically polished in order to remove a defect layer formed by ion implantation or a step which is several to several tens of nanometers in height in the separation plane. In Reference 2, after a separation process, heat treatment such as irradiating a single crystal semiconductor layer with a laser beam or the like is performed, whereby planarity of the single crystal semiconductor layer is improved. In Reference 3, after a separation process, by irradiation with a laser beam, crystal quality of a semiconductor thin film layer is improved, and the semiconductor thin film layer and a transparent insulating substrate are strongly bonded to each other.